1. Scope of the Invention
This invention relates to Data Processing Systems, and more specifically to the ability of providing selected and automatically updated relational database views.
2. Description of the Prior Art
In the fall of 1988 two of the worlds foremost standards organizations, the Consultative Committee on Internal Telephone and Telegraph (CCITT) and the International Standards Organization (ISO), set forth a standard for global Directory Service.
Even though the two organizations collaborated on setting this standard, they each maintain their own name for it. To the CCITT it is knows as CCITT recommendation X.500, whereas for the ISO it is known as International Standard 9594. To simplify the disparate naming for the virtually identical standard, most people refer to it as the Open Systems Interconnection Directory Service--or the OSI Directory.
The architecture of this OSI Directory can be described as a group of cooperating server processes which collectively hold a single global information base about objects of interest and provide read and write access to this information through a query based interface. In essence, the OSI Directory is an iteration of a Distributed Data Base which exhibits some very specific characteristics.
Four characteristics of the OSI Directory which place some constraints on Directory use are:
Access to the Directory Information is through an interactive query/response mechanism. PA1 The scope of distribution for the OSI Directory is global in nature. PA1 The information held about objects tends to be long lived in nature and is thus rarely changed. PA1 All the information for a particular object of interest is maintained in a single Directory entry at only one of the cooperating servers. PA1 A database table, more appropriately known as a relation, corresponds to a set of one or more instances of Directory entries all of the same Object Class. PA1 A database column, more appropriately known as an attribute, exactly corresponds to a Directory entry attribute. PA1 Finally, a database row, more appropriately known as a tuple, corresponds to one instance of a Directory entry.
Because of the access mechanism used and the global nature of the Directory Information base, any service request could be distributed across a number of widely separated but cooperating Directory servers. And real performance constraints can arise when the Directory is used in a local manner. The effect is that any Directory Operation which is targeted to gather information from a number of widely distributed Directory entries can be an extremely time consuming and cumbersome operation to handle in an on-line fashion.
One popular method of eliminating the burden of such lengthy request times is to provide an adjunct process which issues the global queries in a scheduled but off-line fashion. The results are then subsequently stored in a non-volatile cache, typically a local file, and are available for quick local access by the end service consumers. Given the long lived and stable characteristic of the information, the use of a scheduled off-line caching method is an appropriate approach.
The OSI Directory was intended to provide service to a wide range of consumers, each with a special interest. Any particular Directory Service consumer may only be interested in a subset of the total information stored in a particular entry or group of entries.
The single entry characteristic, described above, creates a need for a querying ability which only retrieves selected information. The OSI Directory, through options provided in its query operations (Read & Search) does in fact fulfill this selected information requirement.
As further pertinent background information one must consider the concepts of traditional relational databases. Many traditional relational databases are modelled in terms of tables, columns and rows. And if one were to make an analogy of the traditional relational database model to the OSI Directory Information Base model the following correlations would be true.
A further refinement of relational databases concerns the concept of a database views. Although there is a large body of work published on this particular subject, one such work "Distributed Databases Principals & Systems", by Stefano Ceri and Giuseppe Pelagatti, McGraw-Hill, 1984, pp. 330-332, gives the following classic definition of database views as provided through the industry standard SQL language.
"In SQL/DS, views are defined as the result of an SQL select statement, producing a result relation from one or more operand relations. The result relation gives to the user a new "view" of the database (or external schema in the ANSI-SPARC terminology), which is built through the standard query language."
To paraphrase Ceri and Pelagatti, a view can be likened to a new table formed from a selected subset of other tables. A similar concept can be achieved with the OSI Directory. By using existing OSI Directory query operations in a fashion which corresponds to an SQL select, one can create a new set of related entries from an existing set(s). This new set of selected entries corresponds to a relational database view.